(1) Field of the Invention
The present invention relates to a polyester for a toner to be used for the development of an electronic latent image such as a statically charged image in the electrostatic printing process.
(2) Description of the Related Art
An image-forming process, such as an electrophotographic copying process or an electrostatic printing process, which comprises forming an electronic latent image such as a statically charged image or an electroconductive image corresponding to an image to be recorded on the surface of an electrostatic recording material, developing and visualizing the electronic latent image with a charged toner, transferring the formed toner image on the surface of the electrostatic recording material to a paper or a recording film, and fixing the transferred image, is advantageous in that prints can be obtained at a high speed, the fixing stability of the image on the surface of the recording material is good, and the image-forming apparatus used in this process is easily operated. Accordingly, this image-forming process is widely used in the field of copying machines.
For example, when an image is formed by a photographic copying machine, the surface of a roller composed of a photoconductive and photosensitive material is charged, an electrostatic latent image is formed by the exposure to light reflected from the surface of an original to be copied, the latent image is developed with a toner, the formed visible image is transferred to a paper or the like, and the transferred image is fixed on the paper by compression under heating, and thus a copy print is obtained.
Toners comprising a crosslinked polyester as the binder have been developed for the above-mentioned image-forming process, and toners of this type are disclosed, for example, in U.S. Patent No. 3,938,972, British Patent No. 2,082,788A, Japanese Examined Patent Publication No. 61-59333, and Japanese Unexamined Patent Publication No. 62-127752.
To increase the operational efficiency of copying machines or printers, the image-forming treatment should be carried out at a higher speed, and accordingly, various attempts have been made to increase the treatment speed.
A serious problems when carrying out an image-forming treatment at a high speed in the above-mentioned image-forming process is how to increase the speed of fixing a toner image, formed by visualizing an electrostatic latent image, to the surface of a recording material. To solve this problem, the toner must have an excellent low-temperature fixing property and an excellent electric characteristic such that the electrostatic latent image formed on the surface of the photoconductive and photosensitive material is clearly visualized, but none of the conventional toners comprising a polyester as the binder satisfies these requirements.
The low-temperature fixing property of the toner can be improved by using a polyester binder having a low softening point, but a toner comprising a polyester having a low softening point as the binder is unsatisfactory in that, since the transferability of the toner at the fixing treatment is poor, the toner remains on the surface of a transferring hot roller or the like, and an offset phenomenon, i.e., contamination of a following transfer paper of the like, occurs.
As a toner having an improved offset-preventing property, Japanese Examined Patent Publication No. 61-59333 proposed a toner comprising, as the binder, a crosslinked polyester formed by reacting etherified bisphenol A with a dicarboxylic acid to obtain a linear polyester, and reacting this linear polyester with a polyfunctional carboxylic acid having a trivalency or higher valency, such as trimellitic anhydride.
This toner has an excellent offset-preventing property, but since the softening point of the crosslinked polyester constituting the binder of this toner is high, the high-speed fixing property is not satisfactory.
For example, if the copying operation is carried out at a fixing speed of about 50 sheets per minute when using this toner in the electrostatic photographic copying process, the fixing property of the toner is poor and a copy having a satisfactory toner fixing strength cannot be obtained.
A polyester having a reduced softening point, which can be used as a toner having a good offset-preventing property and a good low-temperature fixing property, is disclosed in Japanese Unexamined Patent Publication No. 62-127752. This polyester is a crosslinked polyester in which long-chain aliphatic hydro-carbon units are introduced, but since the glass transition point (Tg) of the crosslinked polyester is too low, in a toner comprising this polyester as the binder, an agglomeration of toner particles occurs during storage, resulting in blocking. Namely, the toner has a poor storage stability.
The image density and contrast of a toner image formed by visualizing a charged latent image formed on the photoconductive and photosensitive material with a toner are greatly influenced by the toner charge quantity. Accordingly, the toner charge quantity is generally adjusted to an appropriate level by the addition of a charge-control agent. This charge-control agent is expensive, and therefore, the development of a binder resin having an excellent self-charge-ability and able to provide an excellent charge-control effect although using a reduced amount of the charge-control agent, is strongly desired, but such a polyester resin has not been developed as yet.
Furthermore, the moisture resistance of the toner is important when ensuring the stability of the image formed by visualizing the electrostatic latent image on the photoconductive and photosensitive material, but a polyester resin capable of imparting a required moisture resistance to the toner has not been developed.